The present invention relates to resin compositions having long pot life and good curability which comprise polyepoxides, uretdiones and tetraphenyl borate-amine complexes. The present invention also relates to thermoset resins having isocyanurate rings and oxazolidone rings in the structure which are obtained by curing the resin compositions.
With an increase in capacity of electrical apparatus and electronic devices and the demands on small-sized apparatus including the need for high reliability, heat resistant, electrical insulating materials for such apparatus and devices are becoming more and more important. Hetero-ring polymers such as polyimides are known as heat resistant resins. Since most of the conventional hetero-ring polymers were the condensation-polymerization type, which evolve volatile substances upon curing or polymerization, moldability thereof was not good and such polymers were considered as improper for molding, laminating or powder materials.
Silicone resins, another type of a heat resistant resin, have unsatisfactory mechanical strength at high temperatures and have disadvantages such as poor adhesiveness, large water permeability, etc. Applications of conventional silicone resins have necessarily been limited, accordingly.
On the other hand, there have been developed thermosetting resin compositions of the addition polymerization type which comprise polyepoxides and polyisocyanates as disclosed in British patent specification Nos. 843,841; 840,318; 989,390; 982,933; 883,994; and 763,347 and in U.S. Pat. No. 2,594,979. These addition-polymerization type thermosetting resin compositions have eliminated some of the above-mentioned problems, but there was a serious problem that since polyisocyanates are apt to react with water in the atmosphere, the compositions change in their charcteristics at low temperatures such as room temperature and thereby exhibit poor pot life.
U.S. Pat. No. 3,494,888 discloses polymerizable, curable resin compositions comprising an isocyanate-bearing polyisocyanate polymer which has either an isocyanurate ring or a uretdione ring, or both, and a polyepoxide monomer or polymer. According to the disclosure of this patent, the compositions can be cured by heating to temperatures above 200.degree. C. without any additional curing agent, and can also be cured with only short exposure to a higher temperature when polyol or aromatic diamine curing agents are used. These thermosetting resin compositions can generally produce resins which exhibit good heat resistance properties. Various curing agents and curing catalysts are disclosed. From the practical point of view, a curing agent or catalyst is necessary to cure the compositions without exposure to excessive high temperatures for a long period of time. However, a series of experiments by the present inventors have revealed that the compositions disclosed in this patent which contain different types of amines as a catalyst or curing agent showed unsatisfactory pot life in the presence of moisture. It can be said that in this U.S. patent which employs a polyisocyanate polymer having isocyanurate rings or uretdione rings, the resin composition itself has improved pot life, compared with the conventional compositions containing monomeric polyisocyanates because the number of free isocyanate groups in the former is smaller than that in the latter. However, if the composition comprising polyepoxides and uretdiones is admixed with the amine catalysts disclosed, the composition loses pot life in short period of time in the presence of moisture. The mechanism of this phenomenon has not been fully elucidated. However, it can be speculated that dissociation of a uretdione ring of the uretdione compound is accelerated by the conventional amines at lower temperatures thereby increasing the number of isocyanate groups. As a result, the isocyanate groups react with moisture in the atmosphere.